دایره المعارف تاسیسات برق (اطلاعات عمومی برق)
Compensating for Video Loss
Your structured wiring system is likely to have quite a few video ports. In order to get all of these port you'll have to split the source of your video signal quite a few times. Splitters have losses. The more outputs a splitter has the higher the losses. You're likely to have long cable runs. Coax cables have losses. The longer the cable the greater the losses. All of these losses ADD up and the signal level available at your TV or VCR can end up low enough that you'll have poor reception. Take for example my structured wiring system. I have 8 outlet plates so I have an 8-way splitter feeding all of these ports. In addition I have two 2-way splitters that the signal has to pass through before it even gets to the 8-way splitter. See the simplified wiring diagram below.
هر اسپلیتر در هر راه 3.5 دسیبل افت داشته که اعداد قرمز جمع مسیر است
اعداد ابی فقط افت هر خروجی به تنهایی است
A quick note here about dB's before we get too far into this. dB's are a ratio comparing the input level to the output level. And this ratio is expressed as logarithmic ratio of input to output. dB LOSSES are expressed as a NEGATIVE dB and GAIN is expressed as a POSITIVE dB. Rather than get into the mathematical formula for this logarithmic ratio I have put a chart in the Reference page to simplify things. dB's are commonly used to express the ratio in terms of VOLTAGE or POWER. What is important here for video signal levels is the POWER ratio. You'll see from the chart that a -3dB loss means that the output is ONE HALF of the input. And a -6 dB loss means the output is ONE FOURTH of the input. And a -10 dB loss means that the output is ONE TENTH of the input. And a -20 dB loss means that the output is ONE HUNDREDTH of the input. The point here is that seemingly small dB losses are really LARGE signal losses!! Now back to the topic at hand.
In the diagram above each splitter in labeled with it loss level in BLUE text. The 2-way splitters have -3.5dB of loss, the 8-way splitter has -11dB of loss and 100' of RG6/U has -5dB of loss. If you make a diagram like the one above you can ADD up these losses as the signal travels from the source on the left to the destination on the right. This was done in the diagram above and the cumulative signal losses are shown in RED text. As you can see the signal available to a TV or VCR is down -23dB or 1/200th of what the cable company is supplying! This same exercise applies irregardless of what the source of the video is: cable, DSS, modulator or whatever. The idea here is calculate the cumulative amount of LOSS.
Now how much signal loss is acceptable? That depends on how high the signal level is that the cable company is supplying and how sensitive all your TV's and VCR's are. Now most of us don't have the equipment to measure signal levels and our owners manuals for our TV's and VCR are not likely to specify their sensitivity. What I do know is that I DID try my system with the 23dB loss and my reception was very poor - lots of snow and the picture very week.
During the planning stage of my project I saw how high these losses were going to be so I built a +15dB video amplifier into my system. See the wiring diagram below that shows this video amplifier.
As you can see the amplifier has cut the -23dB (1/200th) loss down to -8dB (1/6th). Now, again, how much loss is acceptable? I don't know. But, with this -8dB loss, all of my TV's and VCR's got perfectly good reception.
The point here is to diagram how you plan to wire your system and calculate the losses. Then add a video amplifier that will at least come CLOSE to compensating for that loss.
Shown in the table below are the losses for the components that I bought for my system. Now I bought good components and cheaper components MAY have greater losses.
|100' RG6/U QS||-5dB|
Some things to keep in mind:
- Add in the video amplifier as close to the video source as you can.
- Use a GOOD quality video amplifier with a bandwidth from 50MHz to 1000MHz. Cable modems and set top cable boxes communicate back into the cable system on lower frequencies in the 5MHz to 50MHz range. In order for these devices to work properly the amplifiers and splitters have to have a low frequency 'return path' to accommodate this bi-directional signal path. Amplifiers used for broadcast channel reception with an antenna and conventional CATV (non-digital CATV) don't need this low frequency return path.
- There are a lot of mediocre amplifiers out there so read the specs. I used a Channel Vision amplifier CTV-15PIA. Look for an amplifier with a bandwidth including 40MHz to 1000MHz. Look for one having a 'return path'. A return path is a means for the amplifier to allow signals to pass from the output to the input. Cable TV set top converters and cable modems WILL REQUIRE this return path.
- The same goes for the splitters. Look for a bandwidth from 5MHz to 1000MHz and compare the signal loss levels. I used Channel Vision splitters exclusively. Look for one having a 'return path'. A return path is a means for the splitter to allow signals to pass from the output to the input. Cable TV set top converters and cable modems WILL REQUIRE this return path.
- Video amplifiers come with very limited choices of fixed gains. Adjustable gain amplifiers are available.
- Some of your outlet plates will likely have longer cable runs than others. Unless you have a VERY big home with very long cable runs you can simplify things and just calculate for the longest cable run. Don't forget to include all of the cable runs from the signal source to the signal destination in you calculation.
- Generally speaking one 8-way splitter will have about the same loss as the the equivalent of seven 2-way splitters. The 8-way splitter will be cheaper, smaller and simpler.
- If you want to use a broadcast reception antenna (rooftop, attic, etc.) the very low signal levels you'll get from antennas need to be amplified before they are sent into your wiring system. A 15dB amp will do nicely for this purpose. Use coax cable going to the antenna with a 300 ohm to 75 ohm impedance matching transformer located right at the antenna. The broadcast channels range from 55MHz to 885MHz.
- If you have a cable modem, install it in the same area as your CWP. The ideal configuration will have the cable modem being fed as directly as possible by the cable companies feed. My wiring system, as shown at the top of this page, has the cable companies feed going through just one 2-way splitter before feeding to the cable modem. This is also shown in Overall Wiring Diagram . Cable modems use frequencies far below those used by TV channels. A lot of splitters and video amplifiers do not work well at these lower frequencies.
- Once you get your structured wiring installed it is a good idea to install 75 ohm terminating plugs into all of your video ports or outlets that are not being used. This also goes for splitter output that are not being used. Having unused outputs in a video system will result in reflection due to impedance mismatches. The unused outputs will also radiate or leak RF signals. The 75 ohm terminating plugs are cheap - about $.30 each (except at Radio Shack where they cost $1 each).
- DSS or satellite signals require some special handling especially when you are combining DSS with other types of video. I have plans to add a page or two to this site talk about this soon. Until then you can go to the Home Tech's Learn area for for some good 'How To' information.
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